The present invention relates generally to instruments and methods for measuring flow characteristics, particularly of particulate bearing gases, in a non-invasive manner. The invention pertains particularly to optical tomography of a number of parallel linear elements defining a plane or sheet arranged generally orthogonally with respect to the direction of the flow of the gas in question.
It is desirable in a wide variety of circumstances to measure the characteristics of a flow of gas, particularly one bearing discrete particles within the flow. For example, it is desirable in some circumstances to monitor the particulate content in smokestacks and powder processing plants. It is desirable in other circumstances to measure the total mass flux of particulates in gas streams in some industries, for example, pharmaceutical industries. The information desired includes information on radial and circumferential inhomogeneities in flows such as sprays and particulate laden flows. An example of an area of particular interest is the physical characteristics providing information on the unmixedness of fuel and air in premixed combustors. This information is useful for controlling combustor instability in premixed combustion. Applications include premixed natural gas turbines, automotive engines and power plant combustors. Finally, it is desirable to determine the probability density functions of local absorption coefficients in research applications related to gas flows.
The measurement of such characteristics and the computation of such functions is not simple since the dynamic character of the flow itself results in rapid changes in flow conditions. Thus, it becomes desirable to measure the flow characteristics over a spatial area incorporating at least a substantial portion of the total flow being studied. It is also desirable to measure the flow characteristics repeatedly as a function of time so that the dynamic character of the flow can be faithfully reflected in the measurements of the asymmetry of the flow as well as the probability density functions of local extinction coefficients.
To obtain the probability density functions of local extinction coefficients, three different techniques are available from the prior art. A first technique is based on the discrete probability function method as described in a paper by Y. R. Sivathanu and J. P. Gore (1993), "A Tomographic Method for the Reconstruction of Local Probability Density Functions," J. Quant. Spec. & Rad. Trans., vol. 50, pp. 483-492. A second method is based on Fourier transform of the moments of the measured transmittances as described a paper by M. R. Nyden, P. Vallikul, and Y. R. Sivathanu (1996), "Tomographic Reconstruction of the Moments of Local Probability Density Functions in Turbulent Flow Fields," J. Quant. Spec. & Rad. Trans., vol. 55, pp. 345-356. These two methods require a much longer time for obtaining the local probability density function than is desirable or even available in many of the circumstances outlined previously. A third method based on the image reconstruction algorithm of Vardi and Lee (1992) is preferable for much faster time response. However, this algorithm has so far not been used in the field of turbulent flow deconvolution.
Intrusive probe methods of measuring inhomogeneities in the flow are disclosed in a paper by Dibble, R. W., Chen, J. Y, and Mongia, R. K., 1997, "Optical Probe for Measuring the Extent of Air/Fuel Mixing in Premixed Combustion Turbines," Poster P2, Proceedings of the Annual Program Review Meeting: Advanced Turbine Systems, FETC, Morgantown, WV. However, for obtaining full flow field information, a large number of these probes have to be used making them impractical as full flow field in-homogeneity sensors. Additionally, the presence of the probes themselves contributes changes to the gas flow that in most circumstances is undesirable.
Optical absorption tomography instruments are disclosed in U.S. Pat. No. 4,386,854 and No. 5,798,840. The devices use a fan of laser light and a ring of detectors arranged in a circle to obtain multiple slices though the flow in question. However, the devices are not practical for application in many circumstances since it is often difficult to achieve a ring of detectors around a flow without substantial modification to the flow apparatus, or disturbance of the flow itself. In addition, deconvolution of the absorption data using a ring of detectors is very difficult and thus the repetition rate of such a fan beam tomography system for flow field inhomogeneities is undesirably low. Further, the fiber-optic fan-beam sources employed to illuminate the sample flow typically have a Gaussian distribution of energy across the beam that is undesirable since the signal incident on the detector pixels in the ring array varies from the center to the outermost edge of the beam. If not corrected, this distribution of energy effectively prohibits the amplification of the multiplexed signal from the array since such amplification can easily result in saturating the signal from the center pixel. This distribution of energy, if not corrected, also requires that the signal from the edge have a lower signal to noise ratio. While it has been suggested to smooth this distribution of energy through the use of variable neutral density filters positioned in front of the expanding fan-beam source, such filters have not achieved entirely desirable results. Since deconvolution techniques tend to build up errors towards the center, any variation in the distribution of energy across the beam will also seriously degrade the performance of the system.
U.S. Pat. No. 5,178,002 discloses means for obtaining the density and velocity of a flow field using absorption and/or laser induced fluorescence in conjunction with phase doppler anemometry. While there is a brief reference made to obtaining local extinction coefficient information using multiple transmitters and receivers followed by a tomographic technique, no details are provided. There is no appreciation that the absorption signal varies in time or that, for turbulent flows, tomography from instantaneous measurements would require multiple angles and multiple slices. Further the question of deconvolution of the turbulent flow fields which are not instantaneously axisymmetric has not been addressed at all. There is no disclosure of any use of any form of statistical deconvolution techniques for treatment of the data, or of the problems associated with such techniques. Nor is there any appreciation that the mean transmittance could be used to obtain mean absorption coefficients using conventional and well known non-statistical deconvolution techniques.
U.S. Pat. No. 4,986,654 describes a planar, laser-induced fluorescence method of obtaining time resolved local density and temperature measurements in a flame. This is accomplished by measuring the absorption of fluorescence radiation using multiple cameras to cover multiple angles. The holographic deconvolution technique of Radon transforms mentioned in the patent uses instantaneous absorption along several slices and several projections, and is prone to severe error as pointed out in the Sivathanu and Gore article and the Nyden et. al. article referenced above. Thus, it remains desirable to measure flow characteristics over a spatial area incorporating at least a substantial portion of a total flow being studied as a function of time so that the dynamic character of the flow can be faithfully reflected in the measurements of the asymmetry of the flow as well as the probability density functions of local extinction coefficients.